Photorespiration may be initiated by ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) binding with O2 to produce 2-phosphoglycolate, or by RuBisCo binding with CO2 to produce 3-phosphoglycerate. These two reactions, catalyzed by the same enzyme, are diametrically opposed to each other, thus making photorespiration a very expensive process metabolically. It is estimated that 25% of the carbon fixed by plants during photosynthesis is lost in the process of photorespiration.